Re: Operator Associative in Perl

[ikegami]

there is no associative for '++' and '--' operator

There are two major mistakes there.

• You're actually concerned about the order in which the operands of the list operator (,) are evaluated.

• Operand evaluation order has nothing to do with operator associativity (which determines precedence of instances of operators with the same precedence).

While Perl doesn't document its operand evaluation order, it always evaluates operands from left to right for left-associative operators and exponentiation (**), and from right to left for assignments operators.

This case is no exception. There are two characteristics (features?) of Perl that cause the result you observe:

• Perl passes arguments to functions and subs by reference.

• $a, --$a and ++$a return $a as an l-value.

So, that means

printf("%d%d%d%d%d\n",$a++,$a--,++$a,--$a,$a);

is basically equivalent to

do {
   local @_;
   alias $_[0] = "%d%d%d%d%d\n";
   alias $_[1] = $a++;  # Returns new value 5, $a=6
   alias $_[2] = $a--;  # Returns new value 6, $a=5
   alias $_[3] = ++$a;  # Returns $a (lvalue)  $a=6
   alias $_[4] = --$a;  # Returns $a (lvalue)  $a=5
   alias $_[5] = $a;    # Returns $a (lvalue)  $a=5
   \&printf;
}

When printf gets the values from @_, it sees

• 5
• 6
• 5 (the current value of $a)
• 5 (the current value of $a)
• 5 (the current value of $a)

When I run the same program in C, I got, 45555. Please explain how it is working?

C leaves the operand evaluation order up to the compiler. Results will vary from compiler to compiler. It looks like yours passed by reference, evaluating from right to left.